As last week’s post stated, manufactured fertilisers are reliably the same with every batch, and the same measured applications can be relied on to deliver the same amounts of nutrients every time. Manures, in contrast, are a natural product and thus not so consistent from one ‘batch’ to another, even from the same animals or within the same species. Manures will also differ from species to species, and the aim of this post is to look deeper at those differences, and the pros and cons of these.

One very important detail to be aware of is that manures when used ‘fresh’ are very high in nitrogen. It’s easy to over-apply fertilisers against manufacturers’ recommendations, but even more care needs to be taken with fresh manures simply because of the nitrogen loads they carry.

Nitrogen is an essential macronutrient for plants, but too much can cause more damage than if there were too little in the first place. Too much nitrogen can promote vigorous leaf growth, but at the expense of other growth such as root development or flower and fruit production. Stunted root-development in conjunction with overstimulated leaf growth can lead to a top-heavy and unstable tree more likely to blow over in strong winds if tall enough. Stressed roots are also more susceptible to soil-borne pathogens. And it goes without saying what the consequences of little to no flower and fruit production are!

Too much nitrogen can also remove water from plants, causing them to dehydrate and take on a ‘burnt’ look with yellow or brown leaf edges.

Manures are of two types: ‘cold’ or ‘hot’. ‘Cold’ manures have low levels of nitrogen compared to carbon and can be applied directly to plants without risk of burning. ‘Hot’ manures must always be either ‘aged’ or composted with high carbon materials such as straw or leaves for several months so as to get the nitrogen content down to suitable levels for application.

Aging is to allow the manure to sit undisturbed over time, which in turn allows solids containing nitrogen to break down and release a good proportion of the nitrogen as gas.
Composting is to add carbon-rich materials (straws, leaves) to nitrogen-rich manure, keeping the pile moist and turned regularly until a dark earth-like substance develops.

The more herbivorous the animal, the lower the nitrogen content. The more omnivorous the animal, the higher the nitrogen content. Carnivorous animals such as dogs and cats have the highest nitrogen content of all, but using such manures is not recommended owing to the risk of pathogenic diseases (especially toxoplasmosis from cats).

Herbivores

Of the herbivores, the more grass in the diet (as opposed to woody plants or vegetables), the lower the nitrogen again.

Grass-eating cattle produce manure with the lowest nitrogen content, with a carbon-to-nitrogen ratio of 25:1. Horse manure is next, at 20:1. Both these manures can be applied directly to soil, but at least a month of composting is still recommended. Horse manure is especially likely to be full of weed seeds, which can be rendered infertile from the heat of a composting pile. Composting also helps break down any wormicides or antibiotics that may be present.

Sheep, goats, alpacas and llamas incorporate more woody stems into their diets and have richer manures as a result (around 15:1). These are all best aged for about six weeks, or composted before use. Any straw bedding collected with the manure makes an excellent and instant carbon source for composting. But because these manures come in a pelleted form, applying fresh manure plus straw as a mulch around fruit trees is a safe and easy way to fertilise your trees, as the pellets age in place while the nutrients slowly leach into the soil.

Rabbits have the richest manure of all, owing to their high vegetable diets. The quantities produced are small however, and would be best utilised as a ‘tea’ by soaking in water for a few weeks and applying the diluted liquid as a fertiliser.

Omnivores

Omnivores include poultry and pigs. These manures should never be applied fresh as their nitrogen contents will burn plants. The heat produced by fresh, ‘hot’ manures dug into and decomposing in soil can also damage developing roots. These manures can be aged, but composting will get the nitrogen levels down faster.

Composting also ensures a more consistent end-product, in that you know it’s ‘done’ when it takes on a dark earth-like appearance. It is harder to know when aged manures are ‘done’, as aging is simply a process to allow nitrogen gas to outgas. There is no way to know how much has vented and how much remains in the manure pile without specialised measuring equipment, and the rate of outgassing will also fluctuate depending on the season and temperature. Either age these manures over a year for a more consistent outcome, or compost them, or purchase commercial supplies, where one or the other process has already been done for you. Another advantage in using commercially-supplied manures is that the manufacturers can formulate a more consistent product of known nutrient quantities from batch to batch.

Fresh pig manure has a carbon-to-nitrogen ratio of 12:1 and can be composted in a 1:1 ratio with straw. That of poultry is 7:1, and can be composted in a 1:4 ratio with straw. (In case you’re wondering, the carbon-to-nitrogen ratio of omnivorous humans is around 10:1.)

Pig manure has a high pH and can be used to raise soil pH from acidic to neutral, or neutral to alkaline. Always test your soil pH first if considering using this manure regularly and in large quantities.

Poultry manure is higher in phosphorus than other manures. Phosphorus is essential for flower and fruit production, but too much can interfere with zinc, copper, iron or manganese uptake. This manure can be a good booster prior to flower production if phosphorus hasn’t been applied for a while. It can also made into a ‘tea’ by soaking for several weeks in water and applying as a diluted liquid just prior to flowers forming.

A Summary of Pros and Cons

Fresh manures are excellent sources of macronutrients, but their very high levels of nitrogen can cause harm if not used with care.

Cattle and horse manure can be applied directly to the ground, but are still best aged or composted for at least a month to kill weed seeds and deactivate wormicides and antibiotics.

Sheep, goat, alpaca and llama manure need aging or composting for longer, but their pelleted form comes in handy with regards to fruit trees. These pellets can be mixed with leaves or straw for a ready mulch that not only ages in place, but also slowly releases nutrients into the soil via leaching.

People with pet rabbits can benefit from turning that manure into a ‘tea’, perhaps better reserved for smaller plants such as vegetables and ornamentals.

People with access to fresh pig and poultry manures are best aging these for a full year, or composting with straw until fully broken down, before applying to trees. Pig manure is especially useful if soil pH needs to be raised, while poultry manure’s benefit is its high phosphorus content.

One major advantage of all manures, regardless of origin, is that they encourage the growth of earthworms and soil microbes. These organisms boost soil health and improve soil structure as they incorporate a manure’s organic matter into the soil. This is something no manufactured fertiliser can do, as they are designed to deliver nutrients in carefully calculated amounts directly to plants, and nothing more.

Having said that, no manure should be regarded as a complete fertiliser, which is where many manufactured fertilisers do have the advantage, especially those formulated for backyard enthusiasts. But just as commercial orchardists routinely apply different fertilisers with different nutrients and for different purposes throughout a season, so too can people who prefer manures also supplement with additional materials to maximise overall tree health. Just knowing how to recognise nutrient deficiencies and correct for these with applications of trace elements can often be enough — and this topic will be covered in future posts.